Multilayer ceramic capacitors are compact, have a high capacitance, and are highly reliable, and thereby are widely used as electronic components. Recently, multilayer ceramic capacitors have been increasingly required to be further miniaturized, increased in capacitance, less expensive, and highly reliable, according to the development of miniaturized high-performance equipment.
Multilayer ceramic capacitors are usually manufactured by stacking a paste for internal electrode layers and a paste for dielectric layers by a sheet method or a printing method to make a laminate, and then firing the laminate to simultaneously fire the internal electrode layers and the dielectric layers.
The conductive material for the internal electrode layers is generally Pd or a Pd-alloy, but Pd is expensive. Consequently, base metals such as Ni and Ni-alloys, which are relatively inexpensive, have been used. However, when a base metal is used as a conductive material for internal electrode layers, oxidization of the internal electrode layers is caused by the firing in an atmosphere. Therefore, in order to simultaneously fire dielectric layers and internal electrode layers, the firing must be conducted in a reducing atmosphere. However, when the firing is conducted in a reducing atmosphere, reduction of the dielectric layers occurs and thereby the specific resistance is decreased. Consequently, non-reducing dielectric materials have been developed.
However, in a multilayer ceramic capacitor using a non-reducing dielectric material, the insulating resistance (IR) is significantly deteriorated by the application of an electric field (i.e., the IR lifetime is short), and therefore the reliability of such a capacitor is low; which is a problem.
In addition, the relative dielectric constant ∈r decreases with time under the application of a direct electric field; which is also a problem. Furthermore, a capacitor may be used under the application of a direct-current voltage. Generally, the application of a direct-current voltage causes problems to a capacitor having a dielectric material primarily composed of a ferroelectric, i.e., the dielectric constant changing characteristics depending on the applied direct-current voltage (DC bias characteristics), and the capacitance-temperature characteristics (Tc bias characteristics) under the application of a direct-current voltage decrease. In particular, the electric field applied to dielectric layers increases under the application of a direct-current voltage, if the thickness of each dielectric layer is decreased for reducing the size and increasing the capacitance of a chip capacitor according to recent requirements. Therefore, the amount of change in the relative dielectric constant ∈r significantly increases with time, namely, the capacitance significantly changes with time; and the DC bias characteristics and the Tc bias characteristics decrease. Thus, the problems become more prominent.
In addition, capacitors are required to have excellent temperature characteristics. In particular, constant temperature characteristics must be maintained under strict conditions, depending on the application. Recently, multilayer ceramic capacitors have been used in various electronic devices installed in the engine compartment of an automobile, for example, an engine control unit (ECU), a crank angle sensor, and an antilock break system (ABS) module. Since these electronic devices are for stable engine control, drive control, and break control, the circuits must have excellent temperature stability.
It is predicted that the temperature of the environment in which these electronic devices are used decreases to about −20° C. or less in winter in a cold district and increases to about +130° C. or more after starting the engine in summer. Recently, there has been a tendency to reduce the number of wiring harness connecting electronic devices with instruments controlled by the electronic devices. Therefore, the electronic devices may be located on the outside of automobiles. Thus, the environment for electronic devices is becoming stricter. Consequently, capacitors used in these electronic devices are required to have stable temperature characteristics over a broad temperature range.
Temperature-compensating capacitors with excellent temperature characteristics are generally made of (Sr, Ca)(Th, Zr)O3-based or Ca(Ti, Zr)O3-based materials. However, these constituents have very low relative dielectric constants (generally 100 or less). Therefore, it will be impossible to produce capacitors having a large capacitance.
Dielectric porcelain composites having high dielectric constants and stable capacitance-temperature characteristics are generally made of compositions containing BaTiO3 as the primary constituent and Nb2O5—Co3O4, MgO—Y, a rare-earth element (such as Dy or Ho), Bi2O3—TiO2 and the like. However, BaTiO3-based materials with high dielectric constants satisfy only the X7R characteristic (ΔC/C is within ±15% at −55 to 125° C.) specified by the EIA standard and therefore cannot be applied to electronic devices of automobiles used in the above-mentioned strict environment. The above-mentioned electronic devices are required to use dielectric porcelain composites satisfying the X8R characteristic (ΔC/C is within ±15% at −55 to 150° C.) specified by the EIA standard.
As regards composites which have high relative dielectric constants, satisfy the X8R characteristic, and allow to be fired in a reducing atmosphere, the present inventors have already disclosed dielectric porcelain composites shown below (for example, see Patent Documents 1 and 2).
The Patent Document 1 discloses a dielectric porcelain composite containing a primary constituent containing barium titanate; a first accessory constituent containing at least one of MgO, CaO, BaO, SrO, and Cr2O3; a second accessory constituent containing silicon oxide as a major constituent; a third accessory constituent containing at least one of V2O5, MoO3, and WO3; a fourth accessory constituent containing an oxide of R1 (wherein R1 is at least one of Sc, Er, Tm, Yb, and Lu); and a fifth accessory constituent containing CaZrO3 or a combination of CaO and ZrO2. In the case of 100 moles of the primary constituent, there are 0.1 to 3 moles of the first accessory constituent, 2 to 10 moles of the second accessory constituent, 0.01 to 0.5 moles of the third accessory constituent, 0.5 to 7 moles of the fourth accessory constituent (wherein the number of moles of the fourth accessory constituent is that of R1 alone), and more than 0 but not more than 5 moles of the fifth accessory constituent.
The Patent Document 2 discloses a dielectric porcelain composite containing a primary constituent containing barium titanate; a first accessory constituent containing an oxide of AE (wherein AE is at least one of Mg, Ca, Ba, and Sr); and a second accessory constituent containing an oxide of R (wherein R is at least one of Y, Dy, Ho, and Er). In the case of 100 moles of the primary constituent, there are more than 0 but not more than 0.1 moles of the first accessory constituent and more than 1 mole and less than 7 moles of the second accessory constituent.    Patent Document 1: Japanese Patent No. 3348081    Patent Document 2: Japanese Patent No. 3341003